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1 INTRODUCTION

This paper is an extension of work originally

presented in conference name [1].

The development of new methods and technical

schemes for measuring three-phase voltage

parameters for controlling autonomous generator sets

is justified by a fundamentally new approach to

controlling their voltage. The need to increase the

speed of the three-phase voltage sensor is caused by

the prospect of using asynchronous generators with

capacitor excitation on ships [2,3]. There was a need

for a voltage sensor that measures the average value

of the three-phase voltage of an autonomous

generator, the speed of which is unstable, for half the

alternating current period.

Improvement of autonomous power plants,

including marine transport facilities, improvement of

their technical and economic indicators is the main

trend in the development of ship's technology. For

example, the capacity of electrical equipment installed

on modern ships reaches 40% of the capacity of a

power plant, therefore, the choice of an optimal

composition of a power plant with improved

operational qualities contributes to an increase in the

efficiency of the power plant.

Currently, on sea-going ships, synchronous

generators are mainly used as sources of electricity,

which is explained by the simplicity of the technical

means for regulating their voltage. The multi-turn

excitation winding on the generator rotor makes it

possible to control the voltage of the synchronous

generator using relatively small currents.

However, this "simplicity" caused two other

problems: the rotating slip rings on the rotor and the

high inductance of the excitation circuit, which

significantly reduced the speed of the voltage

stabilization system.

As a result of numerous scientific and

technological developments of synchronous

generators and their excitation systems, generating

sets have been created that have practically exhausted

the possibilities for further improving the quality of

electricity [4].

Modern brushless synchronous generators with

inverted exciter and rotating diodes have eliminated

rotating contacts but have significantly complicated

the design of the generator. To increase the speed of

Measuring the Voltage of a Three-Phase Circuit in a

Generator Set Control System

L. Vyshnevskyi, M. Mukha, O. Vyshnevskyi & D. Vyshnevskyi

National University "Odessa Maritime Academy", Odessa, Ukraine

ABSTRACT: The article proposes a technical implementation of a high-speed voltage sensor in a three-phase

circuit of a generator set, the frequency of which changes in transient processes. Sequential differentiation,

rectification and intensification of phase voltages allows one to substantially reduce the influence of the current

frequency on the measured voltage during one current period.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 15

Number 4

December 2021

DOI: 10.12716/1001.15.04.21

878

voltage stabilization of synchronous generators,

multiple excitation current forcing systems are used.

Excitation of the asynchronous generator through

the stator circuit allows realizing a voltage regulation

system that is practically invariant from the load, [5-

8].

2 FEATURES OF AUTONOMOUS POWER SUPPLY

SYSTEMS

According to the authors, a radical way to modernize

an electric power plant is the use of asynchronous

generators with a squirrel-cage rotor and capacitor

excitation along the stator circuit, [3].

The voltage control of the asynchronous generator

is carried out on the stator circuit, i.e. along the load

circuit, therefore the inertia of the control and

disturbance channels are the same. As shown by the

research of the authors [2,5,8], this allows to create an

almost load-invariant voltage stabilization system.

The voltage control of the asynchronous generator

is carried out by switching to the three-phase stator

winding of the capacitor banks using triacs at certain

times when the voltages on the capacitor and the

phase winding of the generator are equal. In this case,

capacitors of different phases are switched at different

times, in accordance with the shifts of the phase

voltages. Therefore, the whole process of connecting a

three-phase unit takes time slightly less than the

network period.

2.1 Main part

The features of switching capacitor units determine

the requirements for the speed of the voltage sensor of

a three-phase network. The maximum speed of the

voltage stabilization system requires reliable

measurements of the RMS voltage for the three-

phase current period :

( )

U u t dt



In most of the known sinusoidal voltage sensors,

the average voltage value is measured instead of

the RMS value, which significantly reduces the

computational complexity of their implementation.

With insignificant nonlinear distortions, the RMS

voltage value is proportional to its amplitude ,

therefore such a replacement is quite justified.

The calculation of the voltage average value is

associated with the integration of the voltage

instantaneous value during the positive half-

cycle of the alternating current , [6,9]. In this

case, the fixation of the voltage measurement period is

carried out when the phase voltage passes through

zero.

A significant disadvantage of such sensors is the

dependence of the output signal not only on the

voltage , but also on the frequency of the current

, as shown in our article [1]:

( )

sin cos

cos cos0

m m m

u u t dt U t dt t

U T U T U





  

= =   == −   =



= −  − = =



In marine generator sets with a diesel drive engine,

a significant frequency deviation is allowed, both in

steady-state and in transient modes, [10-12]. In this

case, the instability of the frequency of the current

will introduce significant errors in the output signal of

the voltage sensor, [4].

The article [13] describes and investigates a

method for calculating the average value of an

alternating voltage by sequential differentiation,

rectification, and integration of phase voltages

during a half-period of alternating current :

( )

di m

du t

u dt U



The integration period is set by a

synchronizing pulse which action in during positive

value of the measured voltage. At the end of the

measurement process, the amplitude of the sync pulse

becomes equil zero, the value of the integral is

memorized and stored until the next measurement

period.

Simple circuit solutions of analog voltage sensors

are proposed for both single-phase and three-phase

circuits, which made it possible to significantly reduce

the dependence of the output signal of the voltage

sensor on the current frequency , [5,6], Fig. 1. In

the frequency range from 40 to 50 Hz, the voltage

measurement error is about 2%, at it is -

0.6%, and at it is +0.9%.

Figure 1. Dependence of the output signal of the AC voltage

sensor on the current frequency: – integration of the

rectified voltage; – preliminary differentiation,

rectification, and integration of voltage

A diagram of the analog implementation of the

proposed method for a three-phase network is shown

in Fig. 2.

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Figure 2. Analog part of the sensor for the average voltage

of a three-phase network

Galvanic isolation and lowering of phase voltages

are provided by three transformers TR1, TR2 and TR3

with two output windings having a common point,

which makes it possible to realize full-wave

rectification of the alternating voltage of each phase

using diodes . Differentiation of input

signals is performed by RC - circuits .

Further addition and integration of the measured

signals is carried out by the operational amplifier

, the integration constant of which is determined

by the product of the capacitance on the

resistance (R14, R15). Synchronization with the

mains and discharge of the integrating capacitor is

performed by pulse shunting with the field-effect

transistor , which is controlled by the operational

amplifier . Synchronizing circuit is

connected to the secondary winding of the phase

transformer. Oscillograms of the rectified phase

voltages and the sensor output signal are shown in

Fig. 3.

Figure 3. Oscillograms of the voltage sensor of a three-phase

network

The main advantage of the proposed circuit is the

close to linear waveform at the output of the

operational amplifier at symmetrical phase

voltages. In this case, the slope of the characteristic is

proportional to the amplitude of the phase voltages.

This makes it possible to quickly measure the

amplitude of symmetrical voltages.

However, with unbalanced voltages, the form of

the charge of the capacitor ceases to be strictly

linear, but at the end of the half-period , the

value of the output signal is proportional to the

average value of the unbalanced phase voltages.

When regulating the voltage of an autonomous

generator, the deviation of the average value of the

phase voltages from the given one is used, depending

on which a control action is formed on the excitation

system of the generator.

Therefore, the authors checked the error of

measuring the average value of the asymmetric phase

voltages. The voltage amplitude of one of the phases

varied from 0 to 150% of the nominal value at

100% amplitudes of the other two phases. The

obtained values of the output voltage of the

sensor in relative units were compared with the

theoretical ones shown by the dotted line. These

results are shown in Fig. 4.

Figure 4. Accuracy and error in measuring the average

value of asymmetric phase voltages

When one of the voltages changes from 80 to 120%,

the average value of the three voltage phases changes

from 0.933 to 1.067. In this case, the sensor error is

within the range of 0.1 - 0.6%.

Computer and physical modeling of the proposed

three-phase voltage sensor has shown that with an

asymmetry of the generator phase voltage up to 50%,

the output voltage of the sensor retains a sawtooth

shape, close to linear.

With a complete disconnection of one of the

phases, the error was 3.2%. Oscillograms are shown in

Fig. 5.

Figure 5. Oscillograms of the voltage sensor of a three-phase

network when one of the phases is disconnected

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The voltage waveform in Fig. 5 is already

significantly different from the linear one. The voltage

sensor circuit of a three-phase network remains

operational even when two phases are disconnected,

i.e. for single-phase use. Oscillograms are shown in

Fig. 6.

Figure 6. Oscillograms of the voltage sensor of a three-phase

network when two phases are disconnected (single-phase

mode)

The process of integrating the measured signal is

quite simply implemented in software in the digital

part of the sensor by summation after differentiation

and rectification of signals from phase voltages.

Synchronization of the sensor with the power grid

is also easily implemented by the controller by

comparing the instantaneous signal of one of the

phases with a given constant level.

At the same time, the operations of differentiation

and rectification are implemented by local

radioelements using capacitors, resistors and diodes,

i.e. without ICs containing operational amplifiers.

This circumstance makes it possible to optimize the

ratio of the analog and digital parts of the sensor

according to the criterion of ease of implementation.

Such a diagram of a three-phase voltage sensor is

shown in Fig. 7. Electrical isolation, differentiation

and rectification of phase voltages are performed by

the analog part of the circuit, and integration and

synchronization are performed by the controller,

Fig.8.

Figure 7. Circuit of a three-phase voltage sensor with digital

integration and formation of a synchronization pulse

Transformers TR1-TR3 reduce phase voltages and

provide electrical isolation of power and measuring

circuits. Capacitors C1-C6 and resistors R1, R2, R4, R5,

R7, R8 perform differentiation, and diodes D1-D6

rectify phase voltages. Then the analog signals are fed

to the analog ports A1-A3 of the controller. The

controller interface (terminals 1 to 8) outputs some

bytes of information from the voltage sensor in digital

form, and digital-to-analog conversion of the output

signal of the three-phase circuit voltage sensor is

performed on the resistors R15-R22 and R6.

A resistor-diode divider R10-D7-R11 is used for

synchronizing with the mains voltage, the output

signal of which is put to port A0.

Figure 8. The operation oscillograms of a three-phase

voltage sensor with digital integration and synchronization

with the network

In Fig. 9 shows the operation of the voltage sensor

(green) and the regulator with integral control law

(red).

The controller has a digital integral regulator - pins

9-12. The digital-to-analog conversion of the regulator

signal is performed on resistors R9, R12-R14 and R23.

The setpoint of the regulator input to the controller by

means of the resistors R3 and RV1 via port A4.

Figure 9. An output signals oscillograms of the integral

controller:

a - an increase in the control action with a minimum

intensity

b - decrease in control action with threefold intensity

When simulating and for setting up the program

code, pins 1-8 can be used to output internal program

variables. For example, in Fig. 9 shows the operation

of the voltage sensor and regulator with an integral

881

control law. In Fig. 9, a, b, the upper diagram reflects

the output signal of the integral regulator, and the

lower diagram illustrates the result of software

integration of the sensor signal of a three-phase

circuit.

Digital integration of the sensor signal of a three-

phase circuit and the formation of a synchronizing

pulse allows you to optimize the ratio of the analog

and digital parts of the sensor according to the ease of

implementation criterion.

3 CONCLUSIONS

The developed sensor of the average value of three-

phase voltage measures in half the period of the

alternating voltage.

Preliminary differentiation of phase voltages made

it possible to reduce the dependence of the sensor

output signal on the current frequency by 9 ... 12

times.

The linearity of the sawtooth output of a three-

phase transducer can significantly improve the

performance of the transducer.

The proposed method and scheme for the

implementation of a three-phase sensor remains

operational in case of phase failure and for single-

phase use.

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